1. Field of the Invention
The present invention relates to an image forming apparatus for forming hardcopy images using an electrophotographic process.
2. Description of the Related Art
Electrophotographic processes are widely used in conventional copying machines, facsimiles, page printers and the like as hardcopy image forming means.
Electrophotographic processes comprise a charging process for uniformly charging the surface of a photosensitive member, exposure process for forming an electrostatic latent image on the surface of the photosensitive member in accordance with image data via said exposure process, developing process for forming a toner image by adhering toner to the aforesaid electrostatic latent image, transfer process for transferring said toner image onto a copy sheet, and fixing process for fusing said transferred toner image on the copy sheet.
Image forming apparatus which use electrophotographic processes also use corona chargers provided with a discharge wire. These corona chargers are used as sensitizing chargers, transfer chargers, and separation chargers for separating the copy sheet from the photosensitive member.
Corona chargers are provided discharge wires to which is typically applied a high voltage ranging from about one thousand and several hundred to several thousand volts to accomplish corona discharge. The aforesaid corona discharge supplied a uniform electrical charge to the photosensitive member and the copy sheet.
When the corona discharger is used over a long period of time, toner waste dispersed within the image forming apparatus adheres to the discharge wire, thereby changing the discharge characteristics which in turn reduces the charging efficiency relative to the photosensitive member and copy sheet, and thus produces uneven charged state thereon.
When, for example, a corona charger is used as a sensitizing charger, the change in charging characteristics relative to the photosensitive member causes variations in image density, so that the uneven charging produces irregularities and spotting of the image. Image disturbance due to inadequate transfer or paper jams due to inadequate separation also result when corona chargers are used as transfer chargers or separation chargers.
In order to prevent the aforesaid adverse influences, image forming apparatus are conventionally provided a cleaning means to automatically eliminate the debris adhering to the corona wire.
The period during which the corona wire is cleaned by the cleaning means is generally when power is switched on to the image forming apparatus, or when the corona charger is used and a predetermined number of image formation sheets have been processed.
When the power is switched on to the image forming apparatus, and the warmup period may be used for cleaning the corona wire. However, cleaning of the corona wire will not be accomplished during the period after the power is switched on to the image forming apparatus, and if the apparatus is operated continuously over a long period may result in progressive soiling of the corona wire. Therefore, when the image forming apparatus is to be used continuously over a long period, the power must be switched off temporarily and restarted thereafter. This arrangement is disadvantageous inasmuch as it necessitates a troublesome additional operation for the operator.
When the cleaning period is set by means of the operation time and number of sheets used in image formation, further disadvantages arise inasmuch as the intervals at which cleaning is executed does not necessarily coincide with the intervals at which cleaning is required. That is, the degree to which toner adheres to the corona wire is not necessarily proportional to the operation time and number of sheets used in image formation and will vary in accordance with the B/W ratio (black-to-white ratio) of the original document for image density settings and copies. Accordingly, in practice the discharge wire is not cleaned in spite of adhering toner to the discharge wire thereby reducing the charging characteristics and the wasteful downtime of image forming apparatus is caused during which said apparatus cannot be used (non-operating time for cleaning) when said discharge wire is cleaned while still unsoiled.
As a means of eliminating the aforesaid disadvantages the provision of a suitable sensor has been considered to detect the operating state (operating current and the like) of the corona charger, or the charge state of the photosensitive member and copy sheet. A determination can be made as to the necessity of cleaning the discharge wire based on the aforesaid detection values.
In this arrangement, however, an image forming apparatus having a plurality of corona charger must provide a sensor for each corona charger therein. This arrangement, therefore, has other disadvantages inasmuch as the cost of the image forming apparatus is increased and the control of said image forming apparatus is rendered more complex.